Chapter 15 Changes in Desert Climate

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Chapter 15Changes in Desert Climate
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Is this Climate Change?

• Precipitation deficits lasting months to years?
• lasting several years?
• lasting a decade?
• What is the length of the ENSO cycle? Sun-spot
  cycles?

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Climate Change in Arid Areas Is of Special
Importance

• agriculture
• human consumption

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(Jim Wilson/The New York Times )
(The New York Times )
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Summary of Recent Climate Periods
Pleistocene Epoch (1 M to 10-12 k ybp)
Ice Ages
The Pleistocene period is characterized by
glacial/interglacial cycles (ice ages).
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Summary of Recent Climate Periods
Holocene Epoch (10-12 k ybp to present)
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Methods of Estimating Climate Change

• How do we measure climate change?
• historical records
• meteorological measurements, lake levels, well
  levels, agricultural productivity
• archeological data
• rock engravings, other art, artifacts (e.g.,
  farming implements, hunting and fishing tools),
  abandoned buildings

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Methods of Estimating Climate Change

• How do we measure climate change?
• tree ring studies
• thickness and chemical composition of tree rings
• isolated communities of plants and animals
• crocodiles in the Sahara
• fossils
• bones hippopotamuses in the Sahara
• pollen grains
• remains of dead vegetation
• rodent middens
• ice cores (for the desert?)

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Causes of Climate Change in DesertsCauses versus
effects?

• Drought causes/effects
• - blocking ridge
• - sea-surface temperature anomaly
• Climate causes/effects
• - El Nino
• - Glacial cycles

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Glacial Periods

• Why and how would large-scale wind circulation
  and storm patterns change?
• How would this affect rainfall in the poleward
  side of the subtropical high (such as deserts of
  the southwest US)?
• How would this affect rainfall on the equatorward
  side of the subtropical high?

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One Estimate
Glacial conditions (last ice age)
Present conditions
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Changes Associated With Glaciation
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Active Sand Dunes
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Northward Range of the Monsoon
(present conditions) (ice age conditions)
(very warm period)
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The ENSO Cycle
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Global Precipitation Anomalies
(NASA JPL)
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Changes in Earths Geometric Relationship With
the Sun

• Changes in the ellipticity of Earths orbit
  around the sun (100,000 y)

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Changes in Earths Geometric Relationship With
the Sun

• Changes in the tilt of Earths axis
• precession axis executes conical motion like
  top, (25,000 y period)

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Changes in Earths Geometric Relationship With
the Sun

• Changes in the tilt of Earths axis
• nutation change in the angle of Earths tilt
  (40,000 y period)

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JJA Variation in solar output
Strong monsoon
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Changes in Solar Output

• Many cycles in solar output exist.
• One of the most well-documented is the sunspot
  number - an 11 year period.

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Changes in Solar Output
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Maunder Minimum (coldest extreme of Little Ice
Age)
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Anthropogenic Climate Change

• Warming of the climate system is unequivocal
  (IPCC AR4)

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Anthropogenic Climate Change

• What is known
• 11 of the last 12 years are the warmest on record
  (since 1850).
• Atmospheric water vapor has increased in the last
  two decades (why is this important?)
• Sea levels rose 6.7 inches in the 20th century.
• Arctic sea ice is rapidly declining.

(IPCC AR4, NSIDC)
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Sept. 2007 Arctic Sea Ice Extent
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Arctic Sea Ice Extent
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Anthropogenic Climate Change

• What is known
• CO2 levels have increased significantly from
  preindustrial levels.
• Other gases such as CH4 and N2O have also
  increased.
• The radiative forcing of these gases is very well
  understood.

(IPCC AR4)
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(IPCC AR4 SPM)
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(IPCC AR4 SPM)
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More Information

• www.ipcc.ch
• www.realclimate.org